1. Field of the Invention
This invention relates to electro-optical measurement systems and, more particularly, to apparatus for characterizing the semiconductor properties of a sample using the photoluminescence of the sample upon excitation by a photon source.
2. Description of the Background Art
A photoluminescence (PL) measurement is a non-contact, non-destructive method for characterizing semiconductor materials and device structures, and is extensively used as both a diagnostic and quality control tool in semiconductor research and development as well as in the semiconductor manufacturing environment. Whenever a semiconductor material is excited with high energy photons (higher than the bandgap of the semiconductor) from a laser probe, photoexcitation of carriers is effected. These carriers recombine through different processes and emit optical photons of specific energy and spectral distribution which are characteristics of the recombination processes in the material system and give vital information about the optical quality and the bandgap wavelength of the material. The PL spectra thus give characteristic information about the mechanism and the efficiency of the radiative recombination processes.
In recent years, new growth techniques such as Molecular Beam Epitaxy (MBE) and Metallorganic Chemical Vapor Deposition (MOCVD) have been introduced to grow epitaxial material for very sophisticated optoelectronic devices, such as wafers. To achieve good performance and better yield on the wafers, it is very important to obtain uniform material quality for the entire wafer. Besides, the selective area growth and the shadow mask technique are new methods to spatially change the material composition and thickness for photonic device integration such as modulator-laser integration and laser-beam expander integration. The quality control and process development of optoelectronic material mentioned above require PL measurements with high spatial resolution.
Many different arrangements have been employed to characterize the wafers as well as finished devices. Most of these arrangements employ techniques which require a dedicated apparatus with very sophisticated optical and electronic instrumentation. In a representative version of mapping the PL, a single wavelength detection scheme is used where the PL intensity at a fixed wavelength is measured to ensure quality control of the materials. The mapping over the entire wafer area can either be done by keeping the optics fixed and computer controlling the movement of the sample stage (see, for example, the paper entitled "High-Speed Photoluminescence Mapping of III-V Epitaxial Layers of Light-Emitting Diodes", by W. R. Imler, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 4, December 1995), or by a combination of beam raster and sample stage movement (see, for example, the paper entitled "Wafer Level Testing for Semiconductor Laser Manufacture via Spatially Resolved Photoluminescence", by G. E. Carver et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 4, December 1995). The composite picture over the entire area of interest, e.g., over the entire wafer area or over a device, is obtained by advanced signal/video processing techniques for high throughput in a manufacturing environment. In both these techniques, high spatial resolution can be obtained by suitable focusing optics and by spatially filtering the excitation beam with high precision spatial filters (pin-holes). The spatial resolution is determined by the image of the spatial filter on the sample plane. However, if the emission wavelength shifts from the detection wavelength, mapping of the PL intensity at a fixed wavelength is not representative of the optical quality of the material. The reason for this is that there may be spectral variations across the sample which in a fixed wavelength detection system may appear as PL intensity variations. It is therefore necessary in an improved measurement system to obtain complete spectral information.
In systems which are capable of complete spectral information, the major detection system is composed of a high resolution monochromator, detector arrays, multichannel analyzers, and video imagers, all of which are rather expensive and are often dedicated to a single measurement. Moreover, the excitation source is often a bulky gas laser.
Thus, the art is devoid of a PL apparatus and concomitant methodology wherein the excitation and collection optics are common and remain fixed while the sample is placed on a position-controlled stage and, moreover, wherein the measurement components are of the modular, plug-in type so that the components need not be dedicated to the PL measurement, but can be shared with other apparatus to thereby provide cost benefits.